How to calculate rail emissions

Rail is consistently the lowest-carbon mode for surface journeys — and often the most underreported one. Some travel programmes track flight data carefully, but handle rail as a minor rounding-down, missing both the accuracy requirement under CSRD and the opportunity to demonstrate that sustainable choices are already embedded in the programme. This guide explains how rail emissions are calculated, why the factors vary so significantly by route, and how to account for rail accurately in your Scope 3 Category 6 inventory.

Rail in Scope 3 Category 6

Business rail travel — intercity, high-speed, regional and international — sits in Scope 3 Category 6 of the GHG Protocol alongside flights, rental cars and hotel stays. This includes:

Included in Category 6: intercity rail (e.g. London – Edinburgh), high-speed rail (e.g. Eurostar, TGV, ICE), regional trains for business journeys, and overnight sleeper services.

Not Category 6: daily commuting by rail sits in Scope 3 Category 7 (Employee Commuting) and must be tracked and disclosed separately. Metro and subway travel for business trips can be included in Category 6 where data is available, though it is often immaterial.

How rail emissions are calculated

Rail emissions use the same method as other transport modes:

Method 1: Spend-based

Where only expenditure data is available, a spend-based approach using EEIO factors is possible as a fallback, but is significantly less accurate than distance-based and not recommended for reporting where trip-level booking data can be sourced from a TMC or expense system.

Method 2: Distance based (recommended default)

For rail, the activity data is the number of passenger kilometres — journey distance multiplied by number of passengers. The emission factor is expressed in kg CO₂e per passenger kilometre (pkm) and is specific to the route type, operating country and power source.

For UK routes, apply DEFRA emission factors, updated annually, covering national rail, Eurostar and regional services. For international routes, equivalent national grid-based factors are applied: French routes use SNCF-published data, German routes use Umweltbundesamt factors, and so on. The data inputs required are: origin station, destination station, service type (domestic / international), and number of travellers.

Method 3: Fuel based

The most accurate calculation, but fuel data is often unavailable. If rail represents your most material travel emission category, you should try to use fuel data wherever available.

Why rail emission factors vary so much by route

The single biggest driver of variation is the electricity grid mix used to power the trains. Diesel-powered regional services carry materially higher emission factors than electrified routes. Among electrified routes, the difference between a coal-heavy grid and a nuclear-dominant one is significant.

Sources: DEFRA 2025 for UK; SNCF/Eurostar published data for France/Eurostar. Factors are representative averages; actual values vary by year and specific service.

This variation has a practical implication: a UK-based employee taking the Eurostar to Paris generates roughly one-third of the emissions of the same journey by UK domestic rail per km, because the Eurostar route draws on France's nuclear-dominant grid for most of the journey.

Want to learn more about the challenges of comparing rail emissions across routes? Read our dedicated article.

Rail versus flight: the emissions case for modal shift

For most journeys under 500km, rail emits 70–90% less CO₂e per passenger than the equivalent short-haul economy flight. The costs of the two modes are usually similar, and in many cases, rail is a cheaper alternative.

Here are some examples:

The business case is clearest on routes with direct high-speed rail connections under 4 hours. Threshold rules embedded in travel policy — ‘rail mandatory for journeys where direct train time is under 4 hours’ — have the strongest uptake in travel programmes because the choice is simple and concrete. (Thrust Carbon data)

Rail in your inventory and reporting

Rail emissions are relatively straightforward to include in a GHG Protocol Scope 3 Category 6 inventory once booking data is available. The key steps are: aggregate trip data by route, apply the relevant national or operator emission factor, and sum across the programme.

For CSRD reporting, rail must be included in the Category 6 inventory rather than excluded as immaterial, particularly for programmes where domestic rail is a significant travel mode. Excluding low-carbon modes while including high-carbon ones artificially inflates the emissions intensity of the programme relative to peers.